This invention relates to electric motor driven wiper systems for motor vehicle glass areas and particularly to systems having electric dynamic braking of the motor, high and low speed forward modes of motor operation and a reverse mode of motor operation. More specifically, the invention addresses protection of the motor windings and associated wiring from potentially damaging high current conditions.
In prior art wiper systems having typical dynamic brake features, a normally open dynamic brake shunt circuit is coupled between ground and a forward mode power brush, and a forward mode common brush is coupled to ground. During dynamic braking, the circuit providing power to drive the motor is opened and the shunt circuit is closed so that the motor's back e.m.f. generated at the forward mode power brush as a result of motor rotation is shunted to ground, thus effectuating a quick stop of the motor due to the back e.m.f. counteracting the forward rotation of the motor. High current protection is commonly part of the dynamic brake shunt circuit to prevent motor and/or wiring damage should the shunt circuit close while the circuit providing power is still closed and the motor is running in a forward mode. A fusible link in the shunt circuit designed to burn open before motor and/or circuit damage can occur is a commonly employed type of high current protection.
In prior art systems having reverse mode of operation, such as is associated with a reverse-to-park depressed park feature, reversal of the motor is accomplished by switching motor brush polarity from that of a forward mode of operation. The forward mode common brush is taken from ground potential up to the power source potential and a forward mode power brush, normally at a high potential during forward mode operation, is coupled to ground via a reverse mode ground circuit. Mechanical protection is commonly employed in reverse mode of operation to prevent motor and/or wiring damage should a wiper blade become obstructed before reaching its park position. This type of mechanical protection is known in the art as a snow clutch and is effective to allow the motor armature to continue to rotate when the wiper blades are obstructed thereby preventing stall of the motor and subsequent burnout of the motor windings or wiring due to high current therethrough.
Prior art systems perform satisfactorily as designed but improvements can be made. For example, fusible links or the like placed in a dynamic brake shunt circuit are sacrificial high current protection in that they must be replaced each time one burns open. Dynamic braking is lost until repair is effectuated. Not uncommon is such a fusible link etched onto a circuit board which, should it burn open, necessitates replacement of the entire circuit board. This arrangement adds expense beyond that of the fusible link itself and inconvenience to a repair. A mechanical to park snow clutch is subject to typical mechanical failure modes, increases mechanical complexity of the wiper system and comes with a mass penalty. High current protection which is non-sacrificial, is effective in various modes of motor operation and which reduces mechanical complexity and associated mass is therefore desirable.